Polymeric foams are formed by dispersing gas in polymeric matrices. Due to their outstanding properties, such as light weight, good thermal and electrical insulation, excellent cushioning, good chemical resistance, and low cost, these polymeric foams are widely used in construction, packaging, industry, agriculture, transportation, military and aerospace industries, and their application fields are continually expanding. The market demand for polymeric foams has been increasing rapidly in recent years. The most commonly used polymeric foam materials include rigid and flexible polyurethane (PU) foams, polystyrene (PS) foams, polypropylene (PP) foams, polyvinyl chloride foams and polyethylene (PE) foams.
Polymeric foams exist in various forms, particular in the form of foam sheets, and films. In the production of polymeric foams, molding and extrusion manufacturing processes are commonly used. However, polymeric foam products thus obtained are highly flammable. Taking the extruded polystyrene foam (XPS) as an example, XPS is a good thermal insulating material, thus the XPS boards are often used for building insulation, especially for tall buildings. However, due to their high degree of flammability, every year the economic loss due to the building fire is significant. The controversy over flame retardant requirements becomes more serious. The national legislation has required building materials to meet more stringent flame retardancy requirements. As a result, there is an increasing demand for improving the fire retardant properties of these polymeric foam materials.
A well-known approach for improving the flame retardant properties of the polymeric foams is to add flame retardant additives, including halogenated organic compounds and organic phosphate esters during the foam manufacturing process. For example, hexabromocyclododecane (HBCD) has been widely incorporated into polystyrene foams, and halogenated phosphate ester compounds and phosphate esters have been commonly incorporated as the flame retardants into polyurethane foams.
WO91/19758 describes the limited flame retardancy of HBCD and discloses the use of a mixture of brominated aliphatic compounds, especially HBCD, and brominated aromatic compounds such as decabromodiphenyl ether as flame retardants for PS foams. Another U.S. Pat. No. 6,579,911 discloses using HBCD, phosphate ester or phosphorus compounds and flow promoters to improve the flame retardant efficiency of HBCD. In Chinese Patent No. 99814260.3, flame retardants based on expandable graphite and phosphorus compounds for PS foams are disclosed.
One serious problem associated with adding HBCD into XPS during the foam manufacturing processes is the easy decomposition of HBCD. The decomposition of the HBCD may cause the corrosion of the manufacturing equipment as well as the decrease of the flame retardant properties of the foam product. U.S. Pat. No. 5,639,799 and U.S. Pat. No. 5,717,001 disclose methods of improving thermal stability of HBCD for application in polystyrene foams.
Another problem which must be paid attention to when flame retardants are added into the polymeric foams is that amounts of the flame retardant additives and synergists added must be controlled strictly. As at high loading levels, flame retardant additives will negatively affect the structural qualities and skin qualities of the foams and reduce their strength and insulating properties. The typical loading levels of the flame retardant additives in non-foamed polymeric compositions are normally higher than those in foamed compositions. Therefore, the flame retardant additives used in the polymeric foam compositions must be of high efficiency and small amount. The current available technologies in the art cannot solve this dilemma.
Chinese Patent No. CN200610028214.5 discloses a water-soluble flame retardant that comprises several inorganic salts. The flame retardant is incorporated into the polymeric foams by adding inorganic salts during the foam manufacturing process or by impregnating pre-manufactured polymeric foams into the inorganic salt solution. The flame retardants are thus distributed on the external surface of or inside the polymeric foams. In the prior art, we also found a water-soluble fire-retardant interface coating that comprises several inorganic flame retardants and a polymeric interface agent. The interface coating was applied to the XPS board to enhance flame retardancy of the XPS board. However, the composition of this water-soluble flame retardant is very complicated and the process to apply it is very tedious. Meanwhile, the water-soluble flame retardant normally needs longer drying time, which makes it not suitable for continuous industrial production. The application of the water-soluble flame retardant therefore is limited by the weather. Furthermore, the water-soluble flame retardant will be dissolved in water after applied on the foam surface, thus is not suitable for application on the outdoor heat-insulating materials. Finally, due to the poor compatibility between the inorganic flame retardants and the organic polymeric foam matrix, the flame retardant coating can easily pulverize or peeling and shedding on the surface of polymeric foams, thus affecting the subsequent use of the foams.